1. Field of the Invention
The present invention generally relates to semiconductor nanoparticles (“quantum dots”). More particularly, it relates to semiconductor nanoparticles having capping ligands on their outer surface.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
There has been substantial interest in the preparation and characterization of compound semiconductor particles with dimensions on the order of 2-100 nm, often referred to as quantum dots (QDs) and/or nanocrystals. This is mainly because of their size-tunable electronic, optical and chemical properties. For example, many QDs display relatively strong emission in the visible region of the electromagnetic spectrum. Moreover, the wavelength of light absorbed and emitted is a function of the size of the QD. Because of their unique optical properties, QDs are promising materials for commercial applications as diverse as biological labeling, solar cells, catalysis, biological imaging, light-emitting diodes amongst many new and emerging applications.
To date, the most studied and prepared of semiconductor materials have been the II-VI materials, namely, ZnS, ZnSe, CdS, CdSe, CdTe—most notably CdSe due to its tuneability over the visible region of the spectrum. As mentioned semiconductor nanoparticles are of academic and commercial interest due to their properties, which are unique from the properties of the corresponding crystalline bulk forms of the same semiconductor materials. Two fundamental factors, both related to the size of the individual nanoparticles, are responsible for their unique properties. The first is the large surface-to-volume ratio. As a particle becomes smaller, the ratio of the number of surface atoms to those in the interior increases. This leads to the surface properties playing an important role in the overall properties of small particles. The second factor is that, with semiconductor nanoparticles, there is a change in the electronic properties of the material with the size of the particle. Specifically, the band gap gradually becomes wider as the size of the particle decreases. This change in band gap is because of quantum confinement effects. This effect is a consequence of the confinement of an “electron in a box,” giving rise to discrete energy levels similar to those observed in atoms and molecules, rather than a continuous band as in the corresponding bulk semiconductor material. Thus, for a semiconductor nanoparticle, the “electron and hole” produced by the absorption of a photon are closer together than in the corresponding macrocrystalline material, resulting in non-negligible Coulombic interaction between the electron and hole. This leads to a narrow bandwidth emission that is dependent upon the particle size and composition. Consequently, quantum dots have higher kinetic energy than the corresponding macrocrystalline material and the first excitonic transition (band gap) increases in energy with decreasing particle diameter. Thus, quantum dots with a smaller diameter absorb and emit light of higher energy than do quantum dots with a larger diameter. In other words, the color of light absorbed and emitted can be “tuned” as a function of the particle diameter.
Single core nanoparticles, which consist of a single semiconductor material, tend to have relatively low quantum efficiencies due to electron-hole recombination occurring at defects and dangling bonds situated on the nanoparticle surface that lead to non-radiative electron-hole recombinations. One method to eliminate defects and dangling bonds is to grow a shell of a second semiconductor material having a wider band-gap on the surface of the core particle to produce a “core-shell particle”. The shell semiconductor material preferably has a small lattice mismatch with the core material so that the interface between the two materials is minimized. Core-shell particles separate charge carriers confined in the core from surface states that would otherwise act as non-radiative recombination centers. A common example is ZnS grown on the surface of CdSe cores. Excessive strain can further result in defects and non-radiative electron-hole recombination resulting in low quantum efficiencies.
Several synthetic methods for the preparation of semiconductor nanoparticles have been reported. Early routes applied conventional colloidal aqueous chemistry, while more recent methods involve the kinetically controlled precipitation of nanocrystallites, using organometallic compounds.
Since the optical properties of QDs are size-dependent, it is often desirable to produce populations of QDs with a high degree of monodispersity, i.e., with a high degree of uniformity in the size of the QDs in the population. Also, populations of QDs with a high quantum yield (QY, the ratio of photons emitted to photons absorbed) are desirable. Methods have been reported to produce semiconductor QDs with high monodispersity and with quantum yields greater than 50%. Most of these methods are based on the original “nucleation and growth” method described by Murray, Norris and Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. Murray et al. originally used organometallic solutions of metal-alkyls (R2M) M=Cd, Zn, Te; R=Me, Et and tri-n-octylphosphine sulfide/selenide (TOPS/Se) dissolved in tri-n-octylphosphine (TOP). These precursor solutions are injected into hot tri-n-octylphosphine oxide (TOPO) in the temperature range 120-400° C. This produces TOPO coated/capped semiconductor nanoparticles of II-VI material. The size of the particles can be controlled by the temperature, concentration of precursor used, and length of time of the synthesis. This organometallic route has advantages over other synthetic methods, including near monodispersity and high particle cystallinity.
Cadmium and other restricted heavy metals used in conventional QDs are highly toxic and represent a major concern in commercial applications. The inherent toxicity of cadmium-containing QDs prevents their use in any applications involving animals or humans. For example recent studies suggest that QDs made of a cadmium chalcogenide semiconductor material can be cytotoxic in a biological environment unless protected. Specifically, oxidation or chemical attack through a variety of pathways can lead to the formation of cadmium ions on the QD surface that can be released into the surrounding environment. Although surface coatings such as ZnS can significantly reduce the toxicity, it may not completely eliminate it because QDs can be retained in cells or accumulated in the body over a long period of time, during which their coatings may undergo some sort of degradation exposing the cadmium-rich core.
The toxicity also affects applications including optoelectronic and communication because heavy metal-based materials are widespread in many commercial products including household appliances such as IT and telecommunication equipment, lighting equipment, electrical and electronic tools, toys, leisure and sports equipment. Legislation to restrict or ban certain heavy metals in commercial products has been already implemented in many regions of the world. For example, the European Union directive 2002/95/EC, known as the “Restrictions on the use of Hazardous Substances in electronic equipment” (or RoHS), bans the sale of new electrical and electronic equipment containing more than certain levels of lead, cadmium, mercury, hexavalent chromium along with polybrominated biphenyl (PBB) and polybrominated diphenyl ether (PBDE) flame retardants. As a result of this mandate, manufacturers have had to find alternative materials and develop new engineering processes for the creation of common electronic equipment. In addition, on 1 Jun. 2007, a European Community Regulation came into force concerning chemicals and their safe use (EC 1907/2006). The Regulation deals with the Registration, Evaluation, Authorization and Restriction of Chemical substances and is known as “REACH”. The REACH Regulation imposes greater responsibility on industry to manage the risks from chemicals and to provide safety information on the substances. It is anticipated that similar regulations will be extended worldwide. Thus, there is significant economic incentive to develop alternatives to II-VI QD materials.
Due to their increased covalent nature, III-V and IV-VI highly crystalline semiconductor nanoparticles are more difficult to prepare and much longer annealing times are usually required. However, there are now reports of III-VI and IV-VI materials being prepared in a similar manner to that used for the II-VI materials. Examples of such III-VI and IV-VI materials include GaN, GaP, GaAs, InP, InAs and for PbS and PbSe.
For all of the above methods, rapid particle nucleation followed by slow particle growth is essential for a narrow particle size distribution. All these synthetic methods are based on the original organometallic “nucleation and growth” method by Murray et al., which involves the rapid injection of the precursors into a hot solution of a Lewis base coordinating solvent (capping agent). The addition of the cooler solution lowers the reaction temperature and assists particle growth but inhibits further nucleation. The temperature is then maintained for a period of time, with the size of the resulting particles depending on reaction time, temperature, and ratio of capping agent to precursor used. The resulting solution is cooled followed by the addition of an excess of a polar solvent (methanol or ethanol or sometimes acetone) to produce a precipitate of the particles that can be isolated by filtration or centrifugation. Generally, larger particles precipitate easier than smaller particles. Thus, precipitation provides a means of separating the quantum dots as a function of their size. Multiple precipitation steps are required to achieve narrow particle size distributions.
Fundamentally, these prior art preparations rely on the principle of particle nucleation followed by growth. That principle relies on separation of the nanoparticle nucleation step (at a high temperature) from the nanoparticle growth step (at a lower temperature) to obtain monodispersity. Such separation of steps is achieved by rapid injection of one or both precursors into a hot coordinating solvent (containing the other precursor if not otherwise present), which initiates particle nucleation. The sudden addition of the cooler solution upon injection subsequently lowers the reaction temperature (the volume of solution added is about ⅓ of the total solution) and inhibits further nucleation maintaining a narrow nanoparticle size distribution. This method may work well for small-scale synthesis where one solution can be added rapidly to another while keeping a homogenous temperature throughout the reaction. However, on a larger preparative scale, whereby large volumes of solution are required to be rapidly injected into one another, temperature differentials can occur within the reaction, which can lead to a large particle size distribution. Moreover, the need to perform multiple size-selective purification steps is not practical for the production of large quantities of QDs.
U.S. Pat. Nos. 7,588,828, 7,803,423, 7,985,446, and 8,062,703 (collectively referred to herein as “the seeding patents”), the entire contents of which are hereby incorporated by reference, describe synthetic methods for preparing monodisperse QD populations that do not rely on the hot injection methods and the size-selective purification steps described above. Briefly, the methods involve the use of a molecular cluster “seed” compound that serves as a template for the nucleation of the QD semiconductor material in solution. The cluster compound acts as a seed or nucleation point upon which nanoparticle growth can be initiated. In this way, a high temperature nucleation step is not necessary to initiate nanoparticle growth because suitable nucleation sites are already provided in the system by the molecular clusters. By providing nucleation sites that are more uniform than the nucleation sites employed in the methods described above, the synthesis provides a population of QDs that are essentially monodisperse. A significant advantage of the molecular seeding method is that it can be easily scaled-up.
Regardless of how the QD nanoparticles are prepared, the bonding of the atoms about the outer inorganic surface atoms is incomplete. The surface is populated with highly reactive “dangling bonds”, which can lead to particle agglomeration. These uncoordinated surface atoms may also provide excitons with surface states that can give alternative recombination pathways to radiative emission. Such pathways are undesirable and lead to lower luminescence. Additionally, uncoordinated atoms can be more susceptible to oxidation.
The problems associated with uncoordinated dangling bonds can be partially overcome by passivating (capping) the “bare” surface atoms with protecting organic groups. The capping or passivating of particles not only prevents particle agglomeration from occurring, it also protects the particle from its surrounding chemical environment and provides electronic stabilization (passivation) to the particles. The capping agent is typically a Lewis base compound or other electron-donating compound that binds to surface metal atoms of the outermost inorganic layer of the particle (for example, bound to the outermost shell of a core-shell QD particle). When the QD shell synthesis is performed in an electron-donating solvent, such as TOP/TOPO, the capping agent may simply be solvent molecules adhered to the surface of the QD. In the case that a non-electron-donating solvent is used, electron-rich capping agent may be added to the shell synthesis. For example, if a solvent such as THERMINOL is used for the shelling reaction, it may be necessary to add an electron-donating compound, such as myristic acid to the reaction mixture to provide a capping ligand.
While electron-donating capping ligands provide some stability and surface passivation, many of these ligands only weakly adhere to the surface of QD nanoparticles. Desorption of the capping ligands leave vacancies on the surface that can lead to agglomeration, precipitation, and that are detrimental to the quantum efficiency of the QDs. One way of addressing the problem of weakly bound capping ligands has been to use capping ligands that contain functional groups that have a specific binding affinity for atoms on the surface of the QDs. For example, the sulfur of thiol compounds has an affinity for many of the metal atoms, such as zinc, that are commonly components of QD shell semiconductor materials, such as ZnS and ZnSe. Thus, thiols have been widely used as capping ligands for QDs. But thiol capping ligands can also desorb, leaving problematic vacancies on the QD surface. One probable mechanism for thiol ligand desorption is via the formation of disulfide bonds between neighboring thiols on the QD surface, followed by desorption of the disulfide. Another problem with thiol capping ligands is that, in some cases, steric hindrances may prevent complete surface coverage. In other words, once a certain surface coverage is obtained, additional thiol molecules are sterically prevented from reaching the surface of the QD to bond even though there are still a significant number of dangling bonds at the surface left unfilled. There is thus a need for more affective capping ligands for QDs in order to maximize the performance and stability of QD materials.